Tube and shell heat exchangers come in many forms and sizes. The shell can be a simple cylinder in form and a series of parallel tubes can extend through the cylinder. Openings in the cylindrical shell can flow fluids into, through, and out of the shell while a second fluid is flowed through the tubes. Any differential in temperature between the two fluids causes an exchange of heat through the tubes walls. It will be convenient to refer to the fluid flowing through the tubes as the primary fluid, and the fluid flowing through the shell as the secondary fluid.
Enhancement of the heat exchange can be brought about by periodically mounting baffles extending transverse the axis of the tubes and forming openings through the baffles so that the secondary fluid will be directed to flow transverse the tube axis, weaving its way from the shell inlet to the shell outlet. In this arrangement, the secondary fluid is provided with greater contact time with the external surfaces of the tubes than if no baffling were present.
Of course, the tubes must penetrate the flow-control baffles, and there must be some degree of clearance between the baffle holes and tube surfaces. Thermal expansion and contraction between the baffles and the tubes must be accommodated by this clearance. Obviously, there will be some degree of secondary fluid leakage between these holes and tube surfaces. The baffle structure is a support to the tubes, as well as a guide for the secondary fluid. The velocity of the secondary fluid over the tube surfaces generates transverse forces on the tubes which cause them to vibrate. These vibrations will cause a variation in the leakage of secondary flow through the holes in the baffle plate. This variation in leakage flow is related to the vibratory motion of the tubes.
There are other forms of shell and tube heat exchangers, a dramatic example being the steam generators of nuclear reactors. In the nuclear steam generator, the vertically extended cylindrical shell has a tube sheet and partition close to the lower end, forming two compartments below the tube sheet which are communicated with tubes bent into a U-shape within the upper part of the shell above the tube sheet. In this form of exchanger, the fluid heated by nuclear fission is flowed through the tubes as primary fluid. The fluid flowed through the shell side of the tubes will be referred to as the secondary fluid and is heated by the primary fluid in the tubes. The tubes are supported within the shell by various forms of baffles/frameworks extended transverse the axis of the tubes. Regardless of the exchanger structure size, and the quantity of the fluids brought into heat exchange relationship, the secondary fluid flowing along the outside surfaces of the tubes can reach the velocity resulting in critical vibration of the tube, causing destructive contact with the baffle/framework structure.
It is the present practice to attach vibration-detecting devices to the external surface of the tubes and transmit vibration signals from these devices to a point external the shell. Typically, a piezoelectric transducer in incorporated into what is termed an accelerometer to form this primary element. These primary elements must transmit their output signals through electrical conductors and are, therefore, subject to the hazards of at least temperature and mechanical connection to receivers external the shells. These devices are far too fragile for the severe service of the heat exchanger. A more simple, direct, rugged primary element is required to respond to the fluid flow variations generated by the tube vibration.